If you have ever opened a magnet data sheet, you have probably seen a strange looping curve. That is the magnetization curve — and it is the key to understanding how any magnetic material behaves.
What the curve shows
A magnetization curve (often called a B-H curve) plots the magnetic field applied to a material against the flux density the material produces. When a fresh material is exposed to a rising field, its flux density climbs quickly. Then the curve levels off — the material is saturated. Extra field no longer adds meaningful magnetization. That flat top shows how much magnetic energy the material can store.
From curve to loop
Now reduce the field back to zero. The flux density does not drop along the same path — it stays high, because the material keeps its magnetization. That is the whole basis of the permanent magnet. Push the field in the opposite direction and the curve swings the other way, tracing a closed figure called the hysteresis loop. The magnetization curve is really the first half of that loop.
Why the second quadrant matters
For permanent magnets, the most important part of the loop is the second quadrant — the section where an opposing field tries to demagnetize the magnet. That region tells you how much strength survives, and it defines data-sheet values like remanence and coercivity. Practical tip: when comparing two magnets, look at that part of the curve, not just the headline strength number.
Read on: the hysteresis loop explained → · the four key parameters →